Apparatus for accurately measuring the inclination of equipment with respect to the vertical (as defined by the direction of gravity) are important in areas such as surveying, construction, manufacturing, and equipment operation. One particular type of inclination measuring apparatus capable of detecting inclination in two dimensions is optics-based and detects inclination by measuring the reflection of a light beam off an unconstrained ("free") liquid surface. This is possible because a free liquid surface remains perfectly horizontal (or, more specifically, always lies along the gravitational equipotential surface) regardless of the inclination of the container in which it resides. Conventional optics-based inclination measuring apparatus require separate projection and light receiving lenses, and further require the use of a large number of parts, such as prisms. Consequently, conventional optics-based inclination measuring apparatus tend to be relatively large and costly. In addition, their accuracy depends on the alignment of certain of the optical components therein, and therefore suffers when optical misalignment occurs.
A conventional optics-base inclination measuring apparatus is set forth in U.S. Pat. No. 5,392,112. The apparatus disclosed therein includes a housing containing optical elements and a light source. The optical elements are arranged such that a beam of light from the light source is split into two beams by a half-mirror provided in the light path between the light source and the free surface of a liquid held in a container. One light beam is reflected by the free liquid surface and falls on a line sensor. The other light beam falls on the line sensor without being reflected by the free liquid surface. Thus, when the housing is inclined, the relative positions of the light beams on the line sensor changes. Thus, the inclination angle relative to the liquid surface is determined based on the relative position of the two beams as sensed by the sensor.